Treatment of hydrocarbons and apparatus therefor



A. L. BAKER April 8, 1941.

TREATMENT OF HYDROOARBONS AND APPARATUS I'HEREFO Filed Aug. 31, 1937 2 Sheets-Sheet 1 lillllllllllllil 00 00 0 9 O 5 ..3 m o 2 O 1 o o o o 7 O o 1 0 V" M o o o o 1. o 7 o n /M F 8 z I L2 WWWOOOO OOO KAWMOQ 09% INVENTOR MBE/PTLBA/fE/P BY m4. Hid/0m ATTORNEY April 8, 1941. BAKER 2,237,468

TREATMENT OF HYDROCARBONS AND APPARATUS THEREFOR Filed Aug. 31, 1937 2 Sheets-Sheet 2 FIG. 2.

v 9 HRH/4 me INVENTOR ALBERT L. BAKER ATTORNEY Patented Apr. 8, 1941 UNITED STATES PATENT OFFICE TREATMENT OF HYDROCARBON S AND APPARATUS THEREFOR Application August 31, 1937, Serial No. 161,805

9 Claims.

This invention relates to the treatment of hydrocarbon fluids and particularly to a method and apparatus therefor particularly adapted for the heating of hydrocarbon fluids.

Hydrocarbon fluids are ordinarily heated by passage through elongated zones of restricted cross section, for example, pipe coils, wherein they are heated to the desired temperature by conduction of heat through the boundary surfaces of the heating zones, for example, by the heating of pipe coils by radiation, or by direct contact with hot gases, or both. Furnaces for this purpose ordinarily comprise inner boundary walls along which the pipe coils may be disposed and a source of hot gases from a side of the furnace chamber whereby the pipe coils are exposed to heat by radiation and/or direct contact with the hot combustion gases.

It is an object of the present invention to provide a method and apparatus therefor for heating hydrocarbon fluids in a furnace containing one or more pipe coils whereby maximum use of the inner surface for the location of tubes is provided and whereby substantially uniform distribution of heat along each tube and in each part of the furnace is effected.

It is a further object of the invention to provide a method and apparatus of the type specified wherein forced draft may be utilized with its resulting advantages without causing restriction of the space available for the location of tubes.

It is a further object of the invention to provide a furnace structure for carrying out the method of the invention, which is compact, affords maximum use of the surface area of the furnace chamber, and permits the use of a shorter stack than in the case of previous structures.

The present invention contemplates a method of heating hydrocarbon fluids passing through tubes or the like positioned along boundary surfaces of a combustion heating zone by firing said zone downwardly from an upper portion thereof relatively close to a side of the zone, by introducing products of combustion, or a combustible material whereby combustion takes place immediately after such introduction, into the combustion zone, and withdrawing the products of combustion from the said zone at an upper portion only of the zone at a point or points: substantially removed from the point or points of introduction. The combustion products pass downwardly through the combustion zone near a side thereof, sweep across the combustion zone for a substantial distance, and then pas upwardly through the combustion zone to the exit. By

this method the tubes containing the hydrocarbon fluids are heated principally by radiation from the hot products of combustion which travel in a stream which first is directed approximately countercurrently to the direction in which the hot gases would normally tend to flow and then concurrent with their natural direction of flow when they are withdrawn from the zone, and at the same time the gases are caused to sweep for a substantial distance across the combustion zone and thereby assist in uniformly dis tributing the radiant heat to all the tube surfaces. Preferably, the invention contemplates the introduction of hot products of combustion or materials of combustion in the manner specified and the Withdrawal of the hot products of combustion from an upper portion of the combustion zone adjacent a side opposite that adjacent which the combustion zone is fired.

The invention further contemplates the provision of a furnace structure of the box type provided with firing means located in an upper portion of the furnace chamber adjacent a side thereof for directing hot products of combustion, or materials of combustion, downwardly into the chamber to effect the desired flow of hot products of combustion downwardly in the chamber adjacent the said side wall, and provided with an exit for the hot products of combustion in an upper portion of the furnace chamber a substantial distance from the point of introduction. The furnace may be fired along one side only of the chamber in which case the exit is preferably located adjacent the opposite side, or the furnace may be fired along a pair of opposite sides of the chamber in which case the exit is preferably located in the upper portion of the chamber intermediate the sides along which the firing is carried out.

The invention further contemplates arrangement of a plurality of burners along the axis of a heating element to afford control of firing across the combustion chamber.

In connection with the above-described method of firing and apparatus therefor a second heating zone may be conveniently provided whereby the hydrocarbon fluids are heated by passage through tubes located in a second or convection chamber through which the hot products of combustion pass after leaving the said combustion chamber. The present invention provides in connection with the above-described apparatus a convection section located above the combustion section over the exit whereby the combustion gases leaving the combustion chamber pass therethrough. The said convection section may be conveniently located in a stack mounted upon the furnace structure for carrying away the combustion gases and providing a draft. Conveniently also an overhead air preheater may be located in the said stack above the convection section whereby'the gases passing from the convection zone are passed in indirect heat exchange with air which is passed to the burners to assist combustion.

The stack mounted on the top of the furnace structure and housing the convection section and the air preheater may be common to two combustion heating Zones which are symmetrically or unsymmetrically arranged with respect to the common convection section. For example, the furnace structure may be fired along a pair of opposite sides, and the exit leading to the stack mounted above the furnace may be located in the roof of the furnace intermediate the sides along which firing is carried out whereby combustion gas from both the combustion zones thereby provided passes upwardly in each combustion zone in the parts located below the exit and into the common convection section.

The accompanying drawings illustrate a furnace embodying the apparatus of the present invention and adapted for use in carrying out the method ofthe present invention. The invention will be further described with reference to the drawings, but it is to be understood that the invention is not limited by such inference but is capable of other embodiments of apparatus adapted to carry out the improved method of the invention. In the drawings, Fig. 1 is a sectional elevation of a furnace constructed in accordance with the invention and adapted to carry out the improved method of the invention for heating hydrocarbon fluids in pyrolytic treatments. Fig. 2 is a sectional side elevation of the furnace illustrated in Fig. 1. In Fig. 2 the left side represents a section taken on line 2-2 of Fig. 1 and the right side represents a section taken along line 2'2' of Fig. 1. In Fig.

2 certain of the tubes in the main heating chamber have been omitted for simplicity.

Referring to the drawings, the furnace I is of the box type having a main heating chamber 2 and a convection section 3. The convection section 3 is arranged above the main heating chamber 2 and is centrally disposed with respect to the side walls 4 and 5 of the main heating chamber 2. The roof of the heating chamber 2 consists of two sections 6 and 1 which are spaced apart to provide a central opening 8 between them by means of which the convection section communicates directly with the interior of the chamber 2. The convection section is iii enclosed by vertical walls 9 and H) which extend upwardly from the adjoining edge of roof portions 6 and 1, respectively. The convection section thus comprises the lowermost portion of the stack for the furnace, the remainder of the stack being arranged above the convection section.

The section of the stack just above convec tion section 3 is enclosed by walls 25,. 26, 21 and 28 which extend upwardly from the top of walls 9 and H3. The front and rear walls 21 and 28, respectively, of this portion of the stack slope inwardly and upwardly to constrict the passage of the gases flowing from the convection section upwardly into the air preheater section.

The air preheater section is defined by the upper portion of walls 25 and 26 and walls 33 and 34 which extend vertically from the upper edge of sloping walls 21 and 28.

Above the preheater section inwardly sloping walls 40, 4|, 42 and 43 form a second constriction in the path of the gases ascending the stack and provide communication between the preheater section and the final and conventional section 32 of the stack which rises vertically from the upper edges of the walls 40, 4|, 42 and 43.

In the combustion chamber 2 banks of tubes such as roof tubes ll, wall tubes I 2 and floor tubes l3 may be arranged along the interior surfaces of the main heating chamber of the furnace. In addition one or more banks of tubes 14 may be arranged in the convection section 3. The tubes are inter-connected in any desired manner whereby hydrocarbons undergoing pyrolytic conversion treatment may be passed through the tubes in single or plural streams and in any one or more desired flows 01' such streams through the furnace and in any desired order. The various tubes may extend through the respective walls of the furnace, and insulating housing l5, shown in Fig. 2, may be provided to cover and protect the cross connections between the ends of the tubes.

The furnace structure illustrating an embodiment of the invention is particularly suited for the treatment of two streams of hydrocarbon fluids simultaneously. The streams may be introduced into a pair of the floor tubes l3 positioned in the center of the furnace and may be passed successively in opposite directions and in single or plural streams through the floor tubes, wall tubes and roof tubes located in the respective right and left halves of the heating chamber 2 and through banks of tubes located in the right and left halves of the convection section. In addition the two streams may be first passed countercurrently to combustion products through banks of tubes located in the right and left halves of the upper portion of the convection section and thereafter introduced into the said centrally located floor tubes.

In the drawings tubes are indicated as ar ranged along the roof, floor, and side walls of the heating chamber. In addition, without departing from the scope of the invention, tubes may be disposed along the end walls also, whereby all six interior Walls of the chamber are utilized for the location of tubes. In addition tubes may be arranged in banks or otherwise, to pass through a combustion section, as Well as along the sides thereof whereby the combustion gases pass through them.

The symmetrical arrangement of the right and left edges of the combustion chamber 2 and the particular location of tubes and the example of the flows described above are given merely for purposes of illustration and may be modified to obtain the desired treatment for the materials being processed. For example, the combustion chamber 2 may be arranged for operation as a single combustion zone with the stack arranged above an end thereof rather than being centrally located as illustrated.

Furthermore, inaddition to the arrangement of tubes along all six boundary surfaces of the heating chamber 2 as illustrated, a bridge wall may be provided in connection with the particular arrangement illustrated located in the center of the main heating chamber below and Such a bridge wall would rest on the floor of chamber 2 and extend upwardly into the opening 8 and terminate near the level of sections 6 and i, or it could be extended to any desired distance short of the level of roof sections 6 and 1. Such a bridge wall would provide a location for additional tubes which could be arranged along one or both faces thereof.

Longitudinally above the roof sections 6 and l of the chamber 2 air ducts l8 and I8" are provided extending substantially the length of the furnace and substantiall parallel to the direction of the roof tubes and are located near the outerwalls A and 5, respectively, of the chamber 2. The air ducts i8 and lil provide for substantially uniform distribution of pressure to a plurality of air ducts l9 and I?! which depend from and communicate with air ducts l8 and I8, respectively. A plurality of air ducts i9 is provided along the length of air ducts it, and each air duct l9 extends through a roof portion 6 of the furnace. A similar arrangement of air ducts l9 connecting with air duct Hi and extending through roof portion 1 is provided. The air ducts l9 and I9 may be arranged in any desired manner with respect to adjacent roof tubes whereby each extends between a pair of adjacent roof tubes or whereby each includes a roof tube in its lower end. For purposes of illustration the air ducts l9 and [9 are shown as arranged along the length of roof tubes He and lib, respectively, whereby each air duct It includes roof tube I la in its lower portion, and each air duct it includes roof tube H1) in its lower portion.

A burner or fuel injector is positioned within each of air ducts l9 and I9 and projects downwardly into the lower portion of each air duct and preferably terminates just above the roof tubes Ha or Mb. The burner or fuel injector it is advantageously of substantially circular cross section at its upper end and tapers in one plane and flares in the plane at right angles therethrough to provide a relatively long narrow fuel outlet in the nozzle which extends along a portion of the length of roof tube Ha or roof tube li'b. Preferably the injector 20 is arranged whereby the longitudinal axis of the outlet portion is parallel to the longitudinal axis of the roof tube above which it terminates. Suitable supporting means such as dependent ears 2! may be provided on the injector 20 to engage the adjacent roof tube lie or Ill) and maintain the burner in proper spaced relation thereto. Fuel lines 22 extend along the length of the furnace portions 6 and l and are connected with the respective burners or fuel injector. 20 by means of 7 suitable pipes and valves 23. A roof portion 24 and end walls it may be provided above the roof sections 6 and i to cover the ducts I8 and i9, feed lines 22 and 23 and roof portions 6 and 1 of the furnace.

In the stack above the convection section and in the preheater section defined by walls 33, 34, 25 and 223 there is located a heat exchanger comprising a lower plate 29 and an upper plate 30 provided with a plurality of holes through which extend hollow pipe line members 3| which provide communication between the portion of the stack below plate 29 and that portion of it above plate en. The walls 33 and 34 are provided, re-

spectively, with openings 44 and 45 which comf' inunicate with lower and upper portions, respectively, of that portion of the stack enclosed between plates 23 and 39. An air intake duct communicates with the opening 44 at one end and with a blower 36 at the other end. Outlet ducts 31 and t8, communicating with the opening 45 at one end and with ducts l8 and I8, respectively, at their opposite ends, are provided. Openings 39 are provided in roof portions 6 and 7 adjacent air ducts i9 and It to permit access to the interior of the chamber 2 for igniting the fuel delivered through burner 20. Shields 39' provided with closure means 46 may be provided for convenience and safety in igniting the burner 2|].

In the arrangement of. tubes and burners shown in the modification illustrated in the drawings the tubes located in the combustion chamber 2 are substantially out of the path of hot products of combustion which sweep through this chamber and are heated predominantly by radiation from the products of combustion. On the other hand, the tubes located in the convection section 3 are located in the path of the hot gases passing out of chamber 2 and are intimately contacted thereby whereby they are heated predominantly by such direct contact. With the furnace constructed and arranged as shown the combustion chamber 2 consists of two radiant heating sections or combustion zones l6 and IT in which the tubes are heated substantially entirely by radiant heat and which are symmetrically disposed with respect to a central overhead convection section 3 which communicates directly with each of the radiant sections I6 and I! at one side thereof.

In the operation of the furnace thus described air is withdrawn from the atmosphere by means of blower 3B and discharged under pressure into duct 35 from which it passes through the preheater section of the stack between plates 29 and 30 and in contact with tubes 3i whereby it is warmed by indirect heat exchange with hot flue gases passing upwardly through the tube 3 l The outlet ducts 3i and 3B supply the heated air under the desired pressure to the ducts l8 and I8, respectively, which in turn feed it to the various ducts I9 and I9, respectively, which extend into the chamber 2. Suitable dampers and other control means may be provided to regulate the quantity and temperature of the air supplied to the ducts l9 and I9.

Suitable fuel, such as gas, is supplied to the burners 20 by means of the fuel lines 22. The fuel supply to the burners may be controlled by any suitable and conventional means. The individual burners may be adjusted by means of valve 23. The fuel is projected from the nozzles of burner 20 in substantially thin wide streams which are elongated in the direction of the roof tubes. Such stream strikes the upper portion of roof tube Ha or lib which deflects it to each side of the tube whereby it becomes intimately mixed with the air passing inwardly through ducts l9 and Iii. The air and fuel are preferably supplied at pressures and velocities whereby ignition and combustion of the mixture preferably takes place at some distance below the roof tubes to avoid excessive localized heating of adjacent tubes.

The natural flow of the hot gases resulting from the combustion is in an upward direction, and this natural tendency is encouraged by providing an overhead stack directly communicating with the chamber 2 wherein the combustion takes place. The furnace is thus fired in a direction which is substantially opposite and countercurrent to the normal flow of gases within the combustion chamber. By firing in this manner the turbulence within the combustion chamber is increased over that existing in furnaces fired in the conventional manner and results in a better distribution of the flame and hot gases within the main heating chamber. The distribution of radiant heat is accordingly more uniform both from end to end of the individual tubes and from one part of the furnace to another. In particular, radiant heat densities at the corners of the furnace will be much higher than in a furnace fired in the conventional manner, which materially increases the efficiency and heating capacity of the furnace. The heat densities in the corners may be further increased and the heat distribution throughout the furnace further controlled by tilting the burners slightly so that the plane of the flame is directed more or less towards the ends 4 and 5, respectively, of the furnace.

The uniform heating of the individual tubes from end to end thereof is assisted by providing burners which are elongated in the direction of the tubes and by providing a plurality of such burners along the length of the tubes. The plurality of burners located along the length of roof tube Ila or Hb may be spaced as desired and cover a substantial portion of the length of the tube. By locating the burners relatively close to the ends of the furnace and by providing the overhead convection section at a central portion of the main heating chamber, the hot gases travel inwardly away from end walls as well as upwardly and thereby tend to sweep the flames inwardly and assist in more uniformly distributing the flames and radiant heat from end to end of the furnace. As hereinabove noted, however, the tubes in the main heating chamber are heated mainly by radiant heat and are located substantially out of the paths of such flames and hot gases.

It is also apparent that the particular embodiment of my improved furnace structure, as illustrated in Fig. 1 and Fig. 2 is wholly symmetrical, which makes possible a better control of the rate of heating of the hydrocarbon fluids undergoing treatment by permitting the symmetric distribution of both tube surfaces and heating flames and gases within the radiant heating sections and the convection section of the furnace.

The location of the stack on the top of the furnace takes advantage of the draft resulting from the height of the furnace and preheater, and thereby permits the use of a considerably shorter conventional stack section 32. This arrangement furthermore permits a substantially smooth, straight line flow of hot gases in the direction of their natural upward flow, thereby decreasing the frictional resistance to such flow of the gases and decreasing the loss of efltlciency and draft resulting from such resistance. This arrangement, together with overhead firing, permits the use of short overhead air ducts and avoids entirely the necessity for underground ducts and flues. The overhead ducts and top firing also permit a ready use of forced draft, which is desirable because it permits the minimum draft to be maintained within the furnace and limits the infiltration of air through the setting.

The invention has been described by reference to an embodiment of the apparatus of the invention employing a pair of symmetrically arranged combustion zones provided with a centrally arranged convection zone communicating with and common to the said combustion zones and in which each part of a unitary furnace structure is arranged to operate in its most efficient manner and cooperate with the other parts in producing an improved apparatus and method for heating hydrocarbon fluids undergoing pyrolytic treatment and which provides increased thermal efiiciency, increased heat capacity, a more uniform distribution of heat and a better control of the rate of heating.

It is to be understood, however, that the invention is not limited to a construction or method employing a pair of combustion zones but is equally well adapted to a construction and method employing a single combustion zone provided with means for firing at one side thereof and communicating exit means at the other side thereof whereby the products of combustion pass down one side of the combustion zone, sweep across the zone and pass upwardly and out of the zone along an opposite side thereof.

If two combustion zones are employed they may be unsymmetrical in arrangement and provided with an overhead stack containing a convection section and air preheater located intermediate but not centrally of the two points of firing.

The advantages resulting from top firing and an overhead convection section could be utilized with a furnace provided with the conventional type of air preheater and stack located at one side of the furnace, or the air preheater could be omitted entirely and the stack located either above or at one side of the furnace. Overhead firing with the advantages of improved control and distribution of heat resulting therefrom could be utilized with a conventional type of furnace in which the convection section is located beside a combustion zone or between two combustion zones and separated from the combustion zone or zones by means of one or more bridge walls extending upwardly from the bottom of the furnace. In the last-mentioned construction the hot products of combustion could be withdrawn from an upper portion of the combustion zone or zones at a side of such zones opposite to that at which the hot products of combustion are introduced. In addition the convection section may be arranged horizontally at a side of the combustion section adjacent the upper portion thereof whereby the hot gases pass upwardly in the combustion chamber as they approach the exit and pass horizontally out of the combustion chamber and through the convection section. However, in the preferred modification of the invention it is advantageous to withdraw the hot products of combustion in the direction of their natural upward flow, as illustrated.

Many other variations and modifications in the improved method and in the structure and arrangement of the apparatus may be made without departing from the spirit and scope of the invention, and it is to be understood that the foregoing description with reference to the specific embodiment does not limit the invention which is capable of other constructions and embodiments.

I claim:

1. The method of heating hydrocarbon fluids to a conversion temperature in a heating apparatus wherein a hydrocarbon fluid is heated predominantly by radiant heat from hot products of combustion while being passed through a plurality of substantially parallel tubes located adjacent a substantially horizontal boundary surface of a box-like combustion chamber, which comprises firing said combustion chamber downwardly into an upper portion thereof near a lateral boundary thereof which is substantially parallel to said tubes, and withdrawing gaseous products of combustion from an upper portion only of said chamber at a part thereof spaced in a direction transverse to said tubes a substantial distance from said lateral boundary near which firing is effected.

2. A method as set forth in claim 1 wherein said downward firing is effected at a plurality of points along the length of said tubes.

3. The method of heating hydrocarbon fluids to a conversion temperature in a heating apparatus wherein a hydrocarbon fluid is heated predominantly by radiant heat from hot products of combustion while being passed through a plurality of substantially parallel tubes located adjacent a substantially horizontal boundary surface of a box-like combustion chamber, which comprises firing said combustion chamber downwardly into an upper portion thereof near a lateral boundary thereof, in a plurality of streams each elongated in horizontal crosssection, said horizontal elongated cross-section being parallel to said tubes and withdrawing gaseous products of combustion from an upper portion only of said chamber at a part thereof spaced in a direction transverse to said tubes a substantial distance from said lateral boundary near which firin is effected.

4. The method of heating hydrocarbon fluids to a conversion temperature in a heating apparatus wherein such fluids are heated predominantly by radiant heat from hot products of combustion while being passed through tubes located adjacent boundary surfaces of a plurality of substantially box-like combustion sections each communicating with each other section at an end thereof and in which at least one such hydrocarbon fluid is additionally heated while being passed through tubes positioned within an overhead convection section communicating with the top of said combustion sections at the communicating ends thereof, which comprises firing said combustion sections downwardly into an upper portion of each near the ends thereof opposite said communicating ends, passing gaseous products of conversion upwardly into said convection section at the communicating ends of the combustion sections, passing said gaseous combustion products upwardly through the convection section and withdrawing said products from the upper portion of said convection section.

5, In a heating apparatus for hydrocarbon fluids comprising a substantially box-like combustion chamber havin a plurality of tubes arranged adjacent a substantially horizontal boundary surface of the chamber for the passage of hydrocarbon fluids therethrough, means arranged longitudinally of said tubes in an upper portion of said chamber near a lateral boundary thereof for firing said chamber in a plurality of downwardly directed streams, and exit means for withdrawing gaseous products of combustion in an upper portion only of said chamber which is spaced in a direction. transverse to said tubes a substantial distance from said firing means.

6. In a heating apparatus for hydrocarbon fluids comprising a substantially box-like combustion chamber having a plurality of tubes arranged adjacent a substantially horizontal boundary surface of the chamber for the passage of hydrocarbon fluids therethrough, a plurality of means arranged longitudinally of said tubes in an upper portion of said chamber near a lateral boundary thereof for firing into an upper portion of the chamber in a plurality of downwardly directed streams each elongated in horizontal cross section in a direction substantially parallel to said tubes, and exit means for withdrawing combustion products in an upper portion only of said chamber which is spaced in a direction transverse to said tubes a substantial distance from said firing means.

7. In a heating apparatus for hydrocarbon fluids comprising a substantially box-like combustion chamber having a plurality of tubes arranged adjacent a boundary surface of the chamber for the passage of hydrocarbon fluids therethrough, means arranged in an upper portion of said chamber near a lateral boundary thereof for firing into the chamber in a downward direction, exit means arranged in the top of said chamber at a substantial distance from said firing means for withdrawing gaseous combustion products in an upward direction, a convection heating chamber arranged above said combuston chamber and communicating therewith through said exit means, a plurality of tubes arranged in said convection heating chamber for the passage of hydrocarbon fluids therethrough, and means for withdrawing gaseous products of combustion from an upper portion of said convection chamber whereby said combustion products pass upwardly through the convection heating chamber.

8. In a heating apparatus for hydrocarbon fluids comprising a plurality of substantially boxlike combustion sections each communicating with each other at an end thereof and having a plurality of tubes arrange-d adjacent a boundary surface of each combustion section for the passage of hydrocarbon fluids therethrough, means for firing downwardly into each of said combustion sections near ends thereof opposite to said communicating ends, exit means for gaseous products of combustion arranged at the top of said combustion sections adjacent the communicating ends thereof, a convection heating chamber arranged above said combustion sections and communicating therewith through said exit means, a plurality of tubes arranged in said convection heating chamber for the pasage of hy drocarbon fluids therethrough, means interconnecting tubes in said convection heating chamber with tubes in at least one of said combustion sections, and means for withdrawing products of combustion from an upper portion of said convection chamber whereby said combustion products pass upwardly through the convection heating chamber,

9. In a heating apparatus for hydrocarbon fluids comprising a plurality of substantially box like combustion sections each communicating with each other at an end thereof and having a plurality of tubes arranged adjacent a boundary surface of each combustion section for the passage of hydrocarbon fluids therethrough, means including air ducts extending through the top of each of said combustion sections near ends thereof opposite to said communicating ends for firing downwardly into upper portions of the combustion sections to heat said tubes, said firing means also including burners, exit means for gaseous products of combustion arranged at the tops of said combustion sections adjacent the communicating ends thereof, a convection heating chamber arranged above said combustion sections and communicating therewith through said exit means, a plurality of tubes arranged in said convection heating chamber for the passage of hydrocarbon fluids therethrough, means intercon- T5 nesting tubes in said convection heating chamher with tubes in at least one of said combustion sections, an air preheating chamber arranged above said convection heating chamber and communicating therewith, a stack arranged above said air preheating chamber and communicating therewith whereby said combustion products pass continuously upwardly through said convection chamber, air preheating chamber and stack,

means for introducing air into said preheating chamber, means within the preheating chamber for passing the products of combustion in indirect heat exchange with said air, and means arranged entirely above said combustion sections for conducting said heated air to said air ducts.

ALBERT L. BAKER. 

